1. Field of the Invention
The present invention relates to a plane transformer having a special shape capable of realizing a reduced size and thickness, which can be used in a power-supply circuit such as a DC-to-DC converter.
2. Description of the Related Art
These days, it is required that electronic apparatuses are smaller in size and thickness with the spread of portable information apparatuses. Accordingly, there is increasing demand that built-in power-supply circuits for such apparatuses be smaller in size and thickness. In particular, it is an important issue to reduce the size and thickness of magnetic devices such as transformers or inductors.
Conventionally, there are known transformers built in this type of electronic apparatuses, such as a transformer which has a toroidal core comprising a primary coil and a secondary coil formed around a magnetic core composed of a ring-shaped magnetic substance formed by winding amorphous alloy foil produced by a quenching method, and another transformer in which a magnetic core (EI type magnetic core) has a shape formed by combining an E type magnetic substance and an I type magnetic substance and the E type magnetic substance include a coil in its central portion.
With the reduction in the size and thickness of electronic apparatuses, there is demand that transformers having a height of several millimeters be used. However, in the transformer having the conventional toroidal core, the magnetic substance thinly formed causes its insufficient strength, which may constitute a problem of strength, and when a ring-shaped magnetic substance formed by winding foil, the width of foil which can be produced is limited, thus, the width of the foil limits the thickness of the ring-shaped magnetic substance, and disadvantageously, there is a certain limit in the decrease in the thickness of the ring-shaped magnetic substance, and the number of turns around the thin substrate is limited.
In the EI core type magnetic core obtained by combining the E type magnetic substance and the I type magnetic substance, the combination of the E type and I type magnetic substances requires a sufficient, overall thickness, and since the thickness of the coil is added, the overall thickness can hardly be set to several millimeters. Further, the conventional transformers and inductors which have exposed coils emit unnecessary high frequency electromagnetic fields, thereby generating electromagnetic noises and causing malfunctions of integrated circuits on the same mother board to which these transformers and inductors are mounted and malfunctions of other apparatuses and devices.
In either of the transformer having the toroidal core and the transformer having the EI core, the magnetic core is needed to be fitted in a resin bobbin. However, when the magnetic core is fitted in the resin bobbin with resin coating, the resin coat and the resin bobbin store heat generated inside the transformer, and cannot emit the heat to the exterior. Thus, disadvantageously, the whole magnetic core readily heats. In addition, the shape of the EI core causes problems of increased core loss due to the magnetic flux concentration and increased copper loss due to intersections of conductor lines and magnetic flux. Thus, defectively, the generated heat concentrates at a point, caused by the construction of the EI core.